Plasmid prophage P1 is one of the simplest genomes that enjoys segregational stability as a consequence of (a) controlled replication, (b) a partition mechanism, and (c) a fail-safe mechanism that can kill cells from which this non-essential genome has been lost. (a). We have established that assaying origin opening by reaction with KMnO4 can be used to study initiation and its control in vivo. The opening requires cooperation of host initiators DnaA and HU, and the plasmid-encoded initiator RepA. The opening was prevented at high origin concentration, but not when RepA concentration was increased within a limit. These results suggest that RepA can be rate-limiting for initiation but a second, as yet unknown, control mechanism is also present. (b) P1 encodes three components of a partition apparatus: proteins ParA and ParB, and a cis-acting site, parS, to which ParB binds. A dysfunction of the P1 partition system has facilitated a mutational analysis of the ParB protein and led us to recognize that ParB can silence genes flanking parS, even several kilobases away. This remarkable effect is reminiscent of gene silencing near eukaryotic centromeres. Hypotheses concerning the nature of the silencing process and its relevance to partitioning are being tested. In collaboration with M. Lobocka, a search for host mutants affecting silencing has been initiated. A simpler partition system, that of the Agrobacterium plasmid pTAR, is also being characterized. (c) P1 encodes an endotoxin (Doc) that can cause death on curing a host of its plasmid and an antidote (Phd) that prevents host death during plasmid maintenance. The relative instability of the antidote means that the cessation of transcription of the phd-doc operon can be lethal. We have characterized an auto-regulatory circuit that dampens dangerous fluctuations in transcription of the operon. Current efforts are directed towards characterizing the target(s) of the toxin.